The present invention relates to digital communication systems and more particularly to digital communication systems employing orthogonal frequency division multiplexing (OFDM).
A point to multipoint wireless communication system represents a potentially effective solution to the problem of providing broadband network connectivity to a large number of geographically distributed points. Unlike optical fiber, DSL, and cable modems, there is no need to either construct a new wired infrastructure or substantially modify a wired infrastructure that has been constructed for a different purpose.
In order to conserve scarce spectrum, the data communication devices of a point to multipoint wireless communication system may all share access to a common frequency. In a typical scenario one or more frequency channels are allocated to downstream broadcast communication from a central access point to a plurality of subscriber units and one or more separate frequency channels are allocated to upstream communication from the subscriber units to the central access point. For upstream communication there is a medium access control (MAC) protocol that determines which subscriber unit is permitted to transmit at which time so as not to interfere with transmission from other subscriber units. For a given upstream frequency, the time domain is divided into frames which are typically of equal duration where each frame represents an individually allocable unit in the time domain. For frames transmitting upstream data, one subscriber unit transmits in each frame. Certain other frames are, however, reserved for access requests by subscriber units. The access request frames are not reserved for any particular subscriber unit in advance. Any subscriber unit wishing to transmit data upstream may transmit its access request in the access request frame. If multiple subscriber units transmit simultaneously in an access request frame there is a collision. The colliding subscriber units will detect the collision and attempt to retransmit their access request after a wait period that is defined pseudo-randomly at each subscriber unit so as to reduce the probability of further collisions.
Orthogonal frequency division multiplexing (OFDM) systems offer significant advantages in many real world communication systems, particularly in environments where multipath effects impair performance. OFDM divides the available spectrum within a channel into narrow subchannels. In a given so-called xe2x80x9cburst,xe2x80x9d each subchannel transmits one data symbol. Each subchannel, therefore, operates at a very low data rate compared to the channel as a whole. To achieve transmission in orthogonal subchannels, a burst of frequency domain symbols are converted from the time domain by an inverse Fast Fourier Transform (IFFT) procedure. To assure that orthogonality is maintained in dispersive channels, a cyclic prefix is added to the resulting time domain sequence. The cyclic prefix is a duplicate of the last portion of the time domain sequence that is appended to its beginning. To assure orthogonality, the cyclic prefix should be as long as the duration of the impulse response of the channel. It is desirable to use OFDM in point to multipoint networks where multipath effects are a concern.
Upstream access requests typically contain very little data. Using an entire frame for an access request from a single subscriber unit is therefore very inefficient. One solution is to multiplex multiple access requests from multiple subscriber units within the same frame in a way that avoids collisions. In an OFDM system one way to accomplish this is to allocate different frequency domain subchannels to different subscriber units so that multiple subscriber units effectively share an OFDM burst and may each transmit their own access request within their own grouping of subchannels. Such a technique is disclosed in U.S. patent application Ser. No. 09/019,938.
To assure maximum performance, an OFDM burst carrying data from a subscriber unit should contain v frequency domain training symbols having predetermined values to assist the receiver in estimating the channel response where vis greater than or equal to the number of symbol periods in a duration of the impulse response from the subscriber unit to the central access point. Symbols used for training are then unavailable for transmission of data. Loss of efficiency due to the inclusion of training symbols is compounded where multiple subscriber units share a single OFDM burst. In the system described in U.S. patent application Ser. No. 09/019,938, each subscriber unit must transmit v training symbols as part of its own grouping within the access request OFDM burst. Thus within a single OFDM burst, there are R*v symbols devoted to training where R is the number of subscriber units sharing the burst. What is needed are systems and methods for more efficiently multiplexing transmissions of multiple subscriber units within the same OFDM burst.
Systems and methods for efficient multiplexing of multiple access requests from disparate sources within a single OFDM burst are provided by virtue of the present invention. Each of multiple subscriber units employ non-overlapping groups of OFDM frequency domain symbols within a single burst for upstream transmission of their access requests. In one embodiment, the OFDM frequency domain symbols are differentially encoded to eliminate the need for upstream transmission of training information. In an alternative embodiment, the group of frequency domain symbols within the burst employed by any particular subscriber unit are contiguous to one another. Channel training for a given subscriber unit need only be performed over the subband occupied by its group of frequency domain symbols. This greatly reduces the number of training symbols required for reception. The reduction or elimination of training symbols increases the number of access requests that may be accommodated within a single burst and/or allows for greater redundancy in transmission of access request data.
One aspect of the present invention provides apparatus for operating a selected data communication device to request access to a shared medium employed by a digital communication system. The apparatus includes a burst forming system that forms frequency domain symbols of an OFDM burst. A first group of frequency domain symbols within the burst includes differentially encoded data and a second group of frequency domain symbols in the burst includes zero values to allow for data transmitted by other data communication devices. The apparatus further includes a converter that transforms frequency domain symbols of the OFDM burst into time domain symbols, and a transmitter system that transmits the time domain symbols as a request for access to the shared medium.